This invention relates to an output split type hydrostatic transmission, and more particularly to an improved hydromechanical transmission having a variable ratio hydrostatic drive unit and a hydraulically controlled differential gear unit to provide a plurality of speed range drives by selective operation of a plurality of clutch engagement mechanisms provided therewith.
There have been introduced some hydromechanical transmissions of this type, wherein the transmission input shafts are directly connected to the differential gear units. With such a hydromechanical transmission as mentioned, certain gears of the differential gear unit have undesired high rotation speed in some drive ranges of high speed ratios only to cause vibrations and noises. Therefore, the rotation speed of the prime engines has to be controlled or a low speed rotation engine such as diesel engine has to be used for the prime power source. Furthermore, in the case that the transmission input torque is given to the differential gear unit through a speed reduction gear train, the speed reduction gear train causes much loss in torque transmission, vibrations and noises in the high speed range drive.